Apparatus and method for achieving symbol timing and frequency synchronization to orthogonal frequency division multiplexing signal

ABSTRACT

A frequency and symbol timing synchronization apparatus for orthogonal frequency division multiplexed (OFDM) signals, and a method performed by the apparatus are provided. This apparatus includes an autocorrelation unit, a comparator, a peak flat detector, a frequency offset estimator, a frequency offset compensation unit, a cross correlation unit and a symbol timing synchronization unit. The autocorrelation unit receives data including a synchronizing symbol made up of at least three identical synchronizing signals, delays the received data by a predetermined delay amount, performs autocorrelation between the received data and the delayed data, normalizes an autocorrelated value, and outputs a normalized autocorrelated value. The comparator compares the normalized autocorrelated value with a predetermined threshold value. The peak flat detector detects as a flat section a section where the normalized autocorrelated value is equal to or greater than the threshold value. The frequency offset estimator estimates a frequency offset within the flat section to obtain a frequency offset value. The frequency offset compensation unit compensates for the frequency offset of a received signal using the frequency offset value. The cross correlation unit performs cross correlation using a frequency offset-compensated signal and a reference signal, and normalizes the cross-correlated value to output a normalized cross-correlated value. The symbol timing synchronization unit detects a point where the cross-correlated value is maximum, and performs symbol timing estimation, thereby performing symbol timing synchronization. In the symbol timing and frequency synchronization apparatus and method, accurate frequency synchronization can be achieved because a large sample error can be allowed. Also, a symbol timing error can be reduced since symbol timing synchronization is achieved using a frequency offset-compensated signal.

TECHNICAL FIELD

The present invention relates to an apparatus and method for achievingsymbol timing and frequency synchronization, and more particularly, toan apparatus and method for synchronizing symbol timing and frequency inan orthogonal frequency division multiplexing (OFDM) system. OFDMtechniques have been adopted as a standard with respect to a physicallayer in 802.11a of IEEE or HIPERLAN TYPE 2 of BRAN ETSI, which are thestandards of a wideband wireless LAN. The present invention relates to afrequency synchronization apparatus and method which is suitable forthis broad-band wireless LAN.

BACKGROUND ART

A conventional OFDM timing and frequency synchronization method isdisclosed in U.S. Pat. No. 5,732,113, issued to Timothy M. Schmidl andDonald C. Cox, entitled “Timing and frequency synchronization of OFDMsignals”. FIG. 1A is a block diagram of the structure of a conventionaltiming and frequency synchronization apparatus disclosed in theabove-described patent, and FIG. 1B is a view for illustrating theoperation of the apparatus of FIG. 1A.

Referring to FIGS. 1A and 1B, in a conventional timing and frequencysynchronization apparatus, a synchronizing symbol having a length of ahalf symbol is made up of two symbols SYN_A, a symbol SYN_B and a symbolSYN_C. A maximum point is detected by autocorrelating between thesynchronizing symbol formed as described above and a delayed symbol. Asymbol timing is obtained from the detected maximum point, and decimalmultiple frequency offset compensation is performed. Then, an inverseFourier transformer inverse-Fourier-transforms a received time-domainsignal and the compensated received signal into a frequency domainsignal. Also, integral-multiple frequency offset compensation isperformed using a differential signal obtained by differentiallyencoding the synchronizing symbols A and B.

However, the above-described conventional method has a problem in thatthe probability of an error occurring during obtaining symbol timing ishigh since a variation in the maximum point of an autocorrelation valueis great due to the influence of noise in a channel. Also, finefrequency synchronization and coarse frequency synchronization depend onsymbol timing synchronization, so that they are sensitive to theinfluence of symbol timing errors. Furthermore, in the above-describedconventional method, a received signal stored in a memory, and a currentreceived signal are both inversely Fourier transformed, which causescomplexity.

Meanwhile, a broad-band wireless LAN uses a 20 MHz frequency band and 64sub-carriers, and a maximum frequency offset is set to be 200 kHz. Thus,a broad-band wireless LAN does not consider a frequency offset whichcorresponds to an integral multiple of a sub-carrier frequency. However,the conventional frequency and symbol synchronization method of OFDMsignals considers an integral-multiple frequency offset, so that it isnot efficient.

DISCLOSURE OF THE INVENTION

An objective of the present invention is to provide a frequency andsymbol timing synchronization apparatus which can acquire more accuratefrequency synchronization and more accurate symbol timingsynchronization from an orthogonal frequency division multiplexed (OFDM)signal which has passed through a multi-path channel to which noise isadded and which causes distortion of amplitude and phase.

Another objective of the present invention is to provide a frequency andsymbol timing synchronization method which is performed in the frequencyand symbol timing synchronization apparatus.

The first objective of the present invention is achieved by a frequencyand symbol timing synchronization apparatus for achieving frequencysynchronization and symbol timing synchronization of an orthogonalfrequency division multiplexed (OFDM) signal, the apparatus including anautocorrelation unit, a comparator, a peak flat detector, a frequencyoffset estimator, a frequency offset compensation unit, a crosscorrelation unit and a symbol timing synchronization unit. Theautocorrelation unit receives data including a synchronizing symbol madeup of at least three identical synchronizing signals, delays thereceived data by a predetermined delay amount, performs autocorrelationbetween the received data and the delayed data, normalizes anautocorrelated value, and outputs a normalized autocorrelated value. Thecomparator compares the normalized autocorrelated value with apredetermined threshold value. The peak flat detector detects as a flatsection a section where the normalized autocorrelated value is equal toor greater than the threshold value. The frequency offset estimatorestimates a frequency offset within the flat section to obtain afrequency offset value. The frequency offset compensation unitcompensates for the frequency offset of a received signal using thefrequency offset value. The cross correlation unit performs crosscorrelation using a frequency offset-compensated signal and a referencesignal, and normalizes the cross-correlated value to output a normalizedcross-correlated value. The symbol timing synchronization unit detects apoint where the cross-correlated value is maximum, and performs symboltiming estimation, thereby performing symbol timing synchronization.

It is preferable that the frequency and symbol timing synchronizationapparatus further includes a mode selection unit for concluding afrequency synchronization mode and selecting a symbol timingsynchronization mode.

Also, preferably, the length of the synchronous signal is equal to orless than the length of an OFDM half-symbol.

It is also preferable that the peak flat detector calculates thedifference or ratio of the autocorrelated value and the threshold valueand detects as a flat section a section where the difference or ratio isequal to or greater than a predetermined value.

Alternatively, the peak flat detector can detect as a flat section asection of a predetermined sample length after a point where theautocorrelated value is greater than the threshold value.

Also, alternatively, the peak flat detector can include an addition unitfor calculating the sum of a predetermined number of samples after apoint where the auto-correlated value is greater than or equal to thethreshold value; and a flat section detection unit for calculating thedifference or ratio of the sum and the threshold value and detecting asa flat section a section where the difference or ratio is greater thanor equal to a predetermined value.

The frequency offset estimator can include a frequency offset estimationunit for obtaining frequency offset values by estimating a frequencyoffset within the section two or more times; and an averaging unit forcalculating the average of the obtained frequency offset values toobtain an average frequency offset value.

The second objective of the present invention is achieved by a frequencyand symbol timing synchronization method for achieving frequencysynchronization and symbol timing synchronization of an orthogonalfrequency division multiplexed (OFDM) signal, the method including: (a)organizing a synchronizing symbol with at least three identicalsynchronous signals; (b) receiving a signal including the synchronizingsymbol, delaying the received signal by a predetermined delay amount,performing autocorrelation between the received signal and the delayedsignal, normalizing an autocorrelated value, and detecting as a flatsection a section where the normalized autocorrelated value is greaterthan a predetermined threshold value; (c) estimating a frequency offsetwithin the flat section to obtain a frequency offset value; (d)compensating for the frequency offset of the received signal using thefrequency offset value; (e) performing symbol timing synchronizationusing a frequency offset-compensated signal and a reference signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of a conventional apparatus for achievingsymbol timing and frequency synchronization of orthogonal frequencydivision multiplexed (OFDM) signals;

FIG. 1B is a view illustrating a symbol timing and frequencysynchronization method which is performed in the symbol timing andfrequency synchronization apparatus of FIG. 1A;

FIG. 2 is a block diagram of an apparatus for achieving symbol timingand frequency synchronization of OFDM signals according to an embodimentof the present invention;

FIG. 3 is a flowchart illustrating a method of achieving symbol timingand frequency synchronization of OFDM signals according to an embodimentof the present invention; and

FIGS. 4A, 4B and 4C are views for illustrating the operation of anapparatus for achieving symbol timing and frequency synchronization ofOFDM signals according to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 2, a method for achieving symbol timing and frequencysynchronization to orthogonal frequency division multiplexed (OFDM)signals, according to an embodiment of the present invention, includes amode selection unit 20, an autocorrelation unit 21, a frequencysynchronization unit 22, a frequency offset compensation unit 23, across-correlation unit 24, and a symbol timing synchronization unit 25.The autocorrelation unit 21 includes a delay unit 212, a complexconjugator 214, a multiplier 216, a moving average calculator 218, and anormalizer 219. The frequency synchronization unit 22 includes acomparator 222, a peak flat detector 224, and a frequency offsetestimator 226. The cross correlation unit 24 includes a reference signalgenerator 242, a complex conjugator 244, a multiplier 246, a movingaverage calculator 248, and a normalizer 249.

FIGS. 4A through 4C are views for illustrating the operation of anapparatus for achieving frequency and symbol timing synchronization ofOFDM signals according to the present invention, and a frequency andsymbol timing synchronization method according to the present invention.This apparatus receives an OFDM signal. The OFDM signal is made up ofpreamble data and payload data. The preamble data include an AGC symboland a synchronizing symbol. The synchronizing symbol used in thisembodiment includes four identical symbols SYNC_A as shown in FIG. 4A.That is, in this embodiment, a synchronizing symbol is made up of foursymbols each having a length of 32 samples, which is half the length, 64samples, of an OFDM symbol, in step 300. It is assumed that an OFDMsignal having this synchronizing symbol is received. Preferably, thelength of the synchronizing symbol is half the length of an OFDM symbol.

The mode selection unit 20 first selects a frequency synchronizationmode. The delay unit 212 delays received data r(K) by a predetermineddelay amount (D) which corresponds to the length, 32 samples, of eachsymbol used during synchronization, in step 302. The complex conjugator214 complex-conjugates delayed data r(K−D). The multiplier 216multiplies the received data r(K) by the delayed data r(K−D), and themoving average calculator 218 calculates a moving average. Here, thesize of a window for the moving average corresponds to the delay amount(D), that is, 32 samples. As described above, the multiplier 216 and themoving average calculator 218 perform autocorrelation, in step 304, andoutput an autocorrelated value. Next, the normalizer 219 normalizes theautocorrelated value, in step 306. Consequently, the autocorrelationunit 21 outputs a normalized autocorrelated value.

The comparator 222 compares the normalized autocorrelated value to apredetermined threshold value, in step 308. The peak flat detector 224detects a section where the normalized autocorrelated value is equal toor greater than the threshold value, as a flat section as shown in FIG.4B, in step 310. The peak flat detector 224 can detect as a flat sectiona section where the difference or ratio between the autocorrelated valueand the threshold value is greater than a predetermined value.Alternatively, the peak flat detector 224 can detect as the flat sectiona section having a predetermined sample length after a point where theauto-correlated value is greater than the threshold value. Also,alternatively, the peak flat detector 224 can be made up of an additionunit (not shown) and a flat section detection unit (not shown). Theaddition unit calculates the sum of a predetermined number of samplesafter a point where the autocorrelated value is greater than thethreshold value. The flat section detection unit (not shown) detects asection where the difference or ratio between the sum and the thresholdvalue is greater than a predetermined value.

The frequency offset estimator 226 estimates a frequency offset withinthe flat section to obtain a frequency offset value, in step 312. Here,frequency offset estimation can be performed at an arbitrary pointwithin the flat section, so that it allows an error of about ±16samples. The frequency offset estimator 226 also outputs a mode controlsignal mode_ctrl received by the mode selection unit 20, when estimationof a frequency offset value is completed. In this way, the frequencysynchronization unit 22 obtains a frequency offset value by estimating afrequency offset within a flat section.

Alternatively, the frequency offset estimator 226 can be made up of afrequency offset calculation unit (not shown)m and an averaging unit(not shown). The frequency offset calculation unit calculates aplurality of frequency offset values within a flat section two or moretimes. The averaging unit obtains an averaged frequency offset value bycalculating the average of the plurality of frequency offset values, andoutputs the averaged frequency offset value as a final frequency offsetvalue.

Following this, the mode selection unit 20 concludes the frequencysynchronization mode in response to the mode control signal and selectsa symbol timing synchronization mode.

The frequency offset compensation unit 23 performs frequency offsetcompensation on a received signal using the final frequency offset valueobtained by the frequency synchronization unit 22.

The reference signal generator 242 generates and outputs a referencesignal, and the complex conjugator 244 complex-conjugates the referencesignal. The multiplier 246 multiplies the complex-conjugated referencesignal by the frequency offset-compensated signal output from thefrequency compensator 23, and the moving average calculator 248calculates a moving average. That is, a cross correlated value isobtained by cross correlation performed by the multiplier 246 and themoving average calculator 248. The normalizer 249 normalizes the crosscorrelated value output from the moving average calculator. In this way,the cross correlation unit 24 performs cross correlation using thefrequency offset-compensated signal and the reference signal andnormalizes a cross correlated value, thereby outputting a normalizedcross correlated value.

The symbol timing synchronization unit 25 detects a point where thecross correlated value is maximum, as shown in FIG. 4C. At this time, anaccurate maximum point can be estimated since a received signal has beenfrequency-compensated. Thus, symbol timing estimation is performed usingthe accurately-estimated maximum point, thereby reducing symbol timingerrors.

As described above, in the symbol timing and frequency synchronizationapparatus and method according to the present invention, frequencysynchronization and symbol timing synchronization are sequentiallyperformed, and an error of about ±16 samples is allowed. That is, alarge sample error can be allowed, so that accurate frequencysynchronization can be achieved. Also, symbol timing estimation isperformed using an accurately-estimated maximum point, thereby reducinga symbol timing error.

As described above, in the symbol timing and frequency synchronizationapparatus and method according to the present invention, accuratefrequency synchronization can be achieved because relatively largesample error can be allowed. Also, a symbol timing error can be reducedsince symbol timing synchronization is achieved using a frequencyoffset-compensated signal.

INDUSTRIAL APPLICABILITY

A symbol timing and frequency synchronization apparatus and methodaccording to the present invention is suitable for a wideband wirelessLAN which does not require a coarse frequency offset estimation, since afrequency offset that is smaller than a symbol spacing is defined.

1. A frequency and symbol timing synchronization apparatus for achievingfrequency synchronization and symbol timing synchronization of anorthogonal frequency division multiplexed (OFDM) signal, the apparatuscomprising: an autocorrelation unit for receiving data including asynchronizing symbol made up of at least three identical synchronizingsignals, delaying the received data by a predetermined delay amount,performing autocorrelation between the received data and the delayeddata, normalizing an autocorrelated value, and outputting a normalizedautocorrelated value; a comparator for comparing the normalizedautocorrelated value with a predetermined threshold value; a peak flatdetector for detecting as a flat section a section where the normalizedautocorrelated value is equal to or greater than the threshold value; afrequency offset estimator for estimating a frequency offset within theflat section to obtain a frequency offset value; a frequency offsetcompensation unit for compensating for the frequency offset of areceived signal using the frequency offset value; a cross correlationunit for performing cross correlation using a frequencyoffset-compensated signal and a reference signal, and normalizing thecross-correlated value to output a normalized cross-correlated value;and a symbol timing synchronization unit for detecting a point where thecross-correlated value is maximum, and performing symbol timingestimation, thereby performing symbol timing synchronization.
 2. Thefrequency and symbol timing synchronization apparatus of claim 1,further comprising a mode selection unit for concluding a frequencysynchronization mode and selecting a symbol timing synchronization mode.3. The frequency and symbol timing synchronization apparatus of claim 1or 2, wherein the length of the synchronous signal is equal to or lessthan the length of an OFDM half-symbol.
 4. The frequency and symboltiming synchronization apparatus of claim 1, wherein the peak flatdetector calculates the difference or ratio of the autocorrelated valueand the threshold value and detects as a flat section a section wherethe difference or ratio is equal to or greater than a predeterminedvalue.
 5. The frequency and symbol timing synchronization apparatus ofclaim 1, wherein the peak flat detector detects as a flat section asection of a predetermined sample length after a point where theautocorrelated value is greater than the threshold value.
 6. Thefrequency and symbol timing synchronization apparatus of claim 1,wherein the peak flat detector comprises: an addition unit forcalculating the sum of a predetermined number of samples after a pointwhere the auto-correlated value is greater than or equal to thethreshold value; and a flat section detection unit for calculating thedifference or ratio of the sum and the threshold value and detecting asa flat section a section where the difference or ratio is greater thanor equal to a predetermined value.
 7. The frequency and symbol timingsynchronization apparatus of claim 1, wherein the frequency offsetestimator comprises: a frequency offset estimation unit for obtainingfrequency offset values by estimating a frequency offset within thesection two or more times; and an averaging unit for calculating theaverage of the obtained frequency offset values to obtain an averagefrequency offset value.
 8. A frequency and symbol timing synchronizationmethod for achieving frequency synchronization and symbol timingsynchronization of an orthogonal frequency division multiplexed (OFDM)signal, the method comprising: (a) organizing a synchronizing symbolwith at least three identical synchronous signals; (b) receiving asignal including the synchronizing symbol, delaying the received signalby a predetermined delay amount, performing autocorrelation between thereceived signal and the delayed signal, normalizing an autocorrelatedvalue, and detecting as a flat section a section where the normalizedautocorrelated value is greater than a predetermined threshold value;(c) estimating a frequency offset within the flat section to obtain afrequency offset value; (d) compensating for the frequency offset of thereceived signal using the frequency offset value; (e) performing symboltiming synchronization using a frequency offset-compensated signal and areference signal.
 9. The frequency and symbol timing synchronizationmethod of claim 8, further comprising the step of concluding a frequencysynchronization mode and selecting a symbol timing synchronization mode.10. The frequency and symbol timing synchronization method of claim 8 or9, wherein the length of the synchronous signal is equal to or less thanthe length of an OFDM half-symbol.
 11. The frequency and symbol timingsynchronization method of claim 8, wherein in the step (b), thedifference or ratio of the autocorrelated value and the threshold valueis calculated, and a section where the difference or ratio is equal toor greater than a predetermined value is detected as a flat section. 12.The frequency and symbol timing synchronization method of claim 8,wherein in the step (b), a section of a predetermined sample lengthafter a point where the autocorrelated value is greater than thethreshold value, is detected as a flat section.
 13. The frequency andsymbol timing synchronization method of claim 8, wherein the step (b)comprises: calculating the sum of a predetermined number of samplesafter a point where the auto-correlated value is greater than or equalto the threshold value; and calculating the difference or ratio of thesum and the threshold value and detecting as a flat section a sectionwhere the difference or ratio is greater than or equal to apredetermined value.
 14. The frequency and symbol timing synchronizationmethod of claim 8, wherein the step (c) comprises: estimating afrequency offset within the flat section two or more times; andcalculating the average of the obtained frequency offset-estimatedvalues to obtain an average frequency offset value.